my definition of loneliness: when you are comparing your existing (or non-existing) human interaction with another experience of human interaction you used to have, or an underlying jealousy for how great the other person's experience of human interaction might be
it's all about comparison, or feeling how your existence is insignificant, and it does sometimes relate to "meaning", etc.
the next question, since it's a comparison, what is loneliness, what is not loneliness, and what's the difference, and how do we find the comparison,